Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)

Abstract

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, zeta(H2), and the ionization fraction, chi(e), in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH+ and HCO+ is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H3+ abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H3+. We apply this technique to the case of the star-forming region DR21(OH), where we successfully detected the (J=3-2) and (J=4-3) rotational transitions of HCNH+. We also determine the C and O isotopic ratios in this source to be 12C/13C=63+-4 and 16O/18O=318+-64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H3+) and chi(e) directly from observations, and estimate zeta(H2) accordingly. Our results, zeta(H2)=3.1x10^(-18) 1/s and chi(e)=3.2x10^(-8), are consistent with recent determinations in other objects.

22 pages, including 3 figures

References in corpus (4)

• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• WFCAM, Spitzer-IRAC and SCUBA observations of the massive star forming region DR21/W75: I. The collimated molecular jets
• Probing the Turbulence Dissipation Range and Magnetic Field Strengths in Molecular Clouds
• The Ionization Fraction of Barnard 68: Implications for Star and Planet Formation

Cited by in corpus (11)

• Cosmic rays in molecular clouds probed by H${}_2$ rovibrational lines -- Perspectives for the James Webb Space Telescope
• Low energy cosmic rays
• Detection of the HC${}_3$NH${}^{+}$ and HCNH${}^{+}$ ions in the L1544 pre-stellar core
• First ALMA maps of cosmic ray ionisation rate in high-mass star-forming regions
• Non-ideal MHD simulations of subcritical prestellar cores with non-equilibrium chemistry
• Astrochemical modelling of infrared dark clouds
• Effect of OH depletion on measurements of the mass-to-flux ratio in molecular cloud cores
• Cosmic-ray-induced H${}_2$ line emission: Astrochemical modeling and implications for JWST observations
• A PDR model for the FIR mid-$J$ CO ladder with universal rotational temperature in star forming region
• Cosmic-ray ionisation rate in low-mass cores: the role of the environment
• Protonated hydrogen cyanide as a tracer of pristine molecular gas